This invention relates to a window type semiconductor laser light emitting device and a process for fabricating the window type semiconductor laser light emitting device.
The internet has rapidly spread far and wide, and the audiographics are drastically improved. The development is achieved on the basis of the large-capacity optical transmission technologies such as, for example, the optical fiber amplification/wavelength division multiplex communication technologies. A low-power consumption low-noise semiconductor laser light emitting device for 1 micron wavelength is used as a light source for optical pumping in the optical fiber amplification technology. When the semiconductor laser light emitting device is employed in the wavelength division multiplex communication system, the wavelength division multiplex communication system requires a high-power semiconductor laser light emitting device, the output power of which is proportional to the amplification ratio.
Crystal degradation called as xe2x80x9coptical abrasionxe2x80x9d is well known to skilled person. The optical abrasion is the phenomenon that the crystal at the end surface of the active layer is degraded due to the high-power laser light output. The optical abrasion proceeds as follows. The laser light is generated in the active layer, and is emitted from the end surface. While the laser light is being radiated from the end surface of the active layer, the surface state at the end surface absorbs part of the laser light, and the active layer generates heat. The heat makes the band gap around the end surface narrower. The narrow band gap absorbs the laser light more than before, and, accordingly, promotes the heat generation. The undesirable feedback loop results in the degradation of the crystal around the end surface, and malfunction takes place in the semiconductor laser emitting device. The optical abrasion is serious in the semiconductor laser light emitting device of 0.6-1 micron wavelength.
A window is effective against the optical abrasion. The window is implemented by using a buried layer wider in band gap than the active layer. The buried layer is transparent to the laser light by virtue of the wide band gap, and forms a window region at the end surface of the active region.
The prior art window type semiconductor laser light emitting device is fabricated as follows. A double heterojunction structure is formed on a semiconductor substrate, and forms a semiconductor laminated structure together with the semiconductor substrate. The semiconductor layers are partially removed from the semiconductor laminated structure for forming the window, and the semiconductor laminated structure is shaped into a striped mesa. The periphery of the striped mesa structure is covered with semiconductor material, which is transparent to the oscillating laser light and electrical insulating. Part of the transparent semiconductor layer serves as the window or the buried layer.
A problem is encountered in the prior art semiconductor laser light emitting device of the type having the striped mesa structure covered with the semiconductor transparent layer in serious coupling loss in the window region. The serious coupling loss is derived from the optical properties of the transparent semiconductor layer. The transparent semiconductor layer does not have any wave-guide in the window region in both of the vertical direction and the horizontal direction. The oscillating laser light is radiated from the internal active region of the resonator toward the boundary between the buried layer and the active layer, and is reflected on the end surface of the resonator. The reflection goes to and comes back in the window region. While the reflection is going to and coming back, the beam shape is deformed, and is coupled to the internal active region, again. When the laser light beam is coupled to the internal active region, energy loss takes place. The energy loss is referred to as xe2x80x9ccoupling lossxe2x80x9d. Increase of the coupling loss results in decrease of the external differential quantum efficiency. Thus, the prior art window type semiconductor laser light emitting device has the problem in that the external differential quantum efficiency is low. Since the boundary is not perfectly perpendicular to the longitudinal direction of the resonator, the laser light beam is bend around the boundary of the buried layer. This results in that the output laser light beam inclines from the longitudinal direction of the resonator toward the vertical direction. Thus, the prior art window type semiconductor laser light emitting device has another problem in that the output laser light beam inclines. Research and development efforts are being made on a solution of the problems.
A solution is disclosed in Japanese Patent Publication of Unexamined Application (laid-open) No. 3-14281. FIG. 1 illustrates the prior art window type self-aligned semiconductor laser light emitting device disclosed therein.
The prior art window type self-aligned semiconductor laser light emitting device is fabricated as follows. On an n-type GaAs substrate 31 are formed an n-type AlyGa1xe2x88x92yAs clad layer 32, an active layer 33, a p-type AlyGa1xe2x88x92yAs clad layer 34 and a p-type GaAs cap layer 35 which are successively grown by using a metal organic vapor phase epitaxial growing technique. Only an active region is covered with a photo-resist mask (not shown), and the p-type GaAs cap layer 35, the p-type AlyGa1xe2x88x92yAs clad layer 34 and the active layer 33 are partially etched away through a wet etching technique. The photo-resist mask is stripped away. The n-type AlyGa1xe2x88x92yAs clad layer is exposed to the space where a window is formed.
Subsequently, a p-type AlzGa1xe2x88x92zAs optical guide layer 38, an n-type AlzGa1xe2x88x92zAs current blocking layer 39 and an n-type GaAs current blocking layer 40 are successively grown on the entire surface of the resultant semiconductor structure. The space is buried with these layers 38/39/40. The layers 38/39/40 in the space are referred to as a window region. The resultant semiconductor structure is partially covered with a photo-resist mask, and the part of the resultant semiconductor structure over the active region is exposed to wet etchant. The p-type AlzGa1xe2x88x92zAs optical guide layer 38, the n-type AlzGa1xe2x88x92zAs current blocking layer 39 and the n-type GaAs current blocking layer 40 are partially etched away in the wet etchant until the p-type GaAs cap layer 35 is exposed, again. The part of the semiconductor structure over the active region is made coplanar with the remaining part of the semiconductor structure or the window region.
An SiO2 stripe 51 is formed on the resultant semiconductor structure, and extends over the window region and the part of the layers 38/39/40 over the active region. Using the SiO2 stripe 51 as an etching mask, the semiconductor structure is shaped into the striped mesa by using a wet etching, and the p-type AlyGa1xe2x88x92yAs clad layer 34 are decreased to 0.3-0.4 micron thick on both sides of the remaining semiconductor structure under the SiO2 stripe 51. An n-type GaAs current blocking layer 41 is selectively grown without removing the SiO2 stripe 51 so as to bury the space on both sides of the remaining semiconductor structure under the SiO2 stripe 51.
Finally, the SiO2 stripe is removed from the part of the semiconductor structure over the active region. A p-type electrode 36 is formed over the resultant semiconductor structure, and an n-type electrode 37 is formed on the reverse surface of the n-type GaAs substrate 31. The resultant semiconductor structure is cleaved along the center of the window region, and the prior art window type semiconductor laser light emitting device is completed as shown in FIG. 1. The p-type AlzGa1xe2x88x92zAs optical guide layer 38 vertically extends in the window region, and the n-type GaAs current blocking layer 41 horizontally extends as similar to the active region for a mode control. The transverse mode light is coupled from the active region to the window region. The window region has wave-guides in both of the vertical and horizontal directions and the transverse mode light is reciprocally propagated in the window region. The beam shape is less deformed, and, accordingly, the coupling loss is small. The oscillating light is guided in the window region, and vertically passes the cleavage. For this reason, the output light beam does not incline.
Other prior art devices are described hereinbelow. Another semiconductor laser light emitting device is disclosed in Japanese Patent Publication of Unexamined Application (laid-open) No. 56-8890. The prior art semiconductor laser emitting device has a pair of epitaxial layers wider in forbidden band width than an active layer, and the active layer is sandwiched between the epitaxial layers. A pair of thick epitaxial layers is grown on one of the epitaxial layers which is closer to the semiconductor substrate than the other in such a manner as to cover a pair of end surfaces of the active layer extending in parallel to the reflecting surface of the resonator. The pair of thick epitaxial layers makes the refractive index large, and realizes an optical wave-guide transparent to the laser light. However, crystal defects are liable to take place in the prior art semiconductor laser emitting device, and the electrodes and the reflecting surface are degraded due to the heat.
Another prior art window type semiconductor laser light emitting device is disclosed in Japanese Patent Publication of Unexamined Application No. 64-42884. The prior art semiconductor laser light emitting device is of the type having a heterojunction, and the transverse mode oscillation is stable. However, the diffraction loss is serous. Accordingly, the threshold current is increased, and the optical damage possibly takes place.
Yet another prior art window type semiconductor laser light emitting device is disclosed in Japanese Patent Publication of Unexamined Application (laid-open) No. 5-67837. The prior art window type semiconductor laser light emitting device has an active layer sandwiched between clad layers, and the active layer and the clad layers form a double heterojunction structure. The active region including the active layer is buried in a buried layer, the forbidden band width of that is greater than that of the active layer. The buried layer provides window regions on both sides of the active region. The window region is formed by plural semiconductor layers different in forbidden band width, the plural semiconductor layers provide a wave-guide. However, it is difficult to spatially match the dispersion of optical intensity in the wave-guide in the active region and the dispersion of optical intensity in the wave-guide in each window region. For this reason, it is impossible to sufficiency increase the coupling efficiency between the window portions and the active region. This results in that a far field pattern is deformed. In case where the wave-guide in the active region is to be offset from the wave-guides in the window regions, the far field pattern is serious due to large wave-guide loss and the deformation of the wave surface. Moreover, the astigmatic difference takes place in both vertical/horizontal directions.
A prior art semiconductor optical device is disclosed in Japanese Patent Publication of Unexamined Application (laid-open) No. 6-112588. The prior art semiconductor optical device includes a first clad layer, an optical wave-guide layer, a quantum well layer and a second clad layer with a mesa structure successively formed on a substrate. The second clad layer is buried in a current blocking layer. Although the reflectivity in the direction of the stripe is decreased, it is difficult to perfectly restrict the Fabry-P rot mode, and the oscillation light is still absorbed at the end surface. Thus, there are various problems in the prior art semiconductor optical devices.
One of the technical goals is to decrease the coupling loss between the wave-guide in the active region and the wave-guide in the window region. In the prior art window type semiconductor laser light emitting device shown in FIG. 1, the oscillating laser light is vertically confined in the p-type Alxe2x80x94zGa1xe2x88x92zAs optical guide layer 38, and the coupling loss is decreased by appropriately locating the p-type AlzGa1xe2x88x92zAs optical guide layer 38 with respect to the active layer 33. However, the semiconductor wafer is dispersed in thickness, and the etching is not precisely controlled. The etching depth to be required is of the order of 2 microns, and the dispersion is never ignoreable. In this situation, it is difficult to precisely control the relative position between the p-type AlzGa1xe2x88x92zAs optical guide layer 38 and the active layer 33 all over the commercial products.
Though not described in detail in Japanese Patent Publication of Unexamined Application No. 3-14281, an etching stopper is effective against the dispersion. Even if the problem inherent in the prior art semiconductor laser light emitting device shown in FIG. 1 is solved by using an etching stopper, another problem is encountered in the scattering loss due to the p-type AlzGa1xe2x88x92zAs optical guide layer 38 left on the sloop between the active region and the window region, and the coupling efficiency in the vertical transverse mode is still low at the boundary between the window region and the active region. This is the first problem.
The location of the bottom surface of the mesa is an important parameter to determine the horizontal light confining structure in the window region. This means that the manufacturer is required to exactly locate the mesa structure at a target position. As described hereinbefore, when the mesa structure is formed, the window region is covered with the photo-resist mask, and the uncovered portion is exposed to the wet etchant. The compound semiconductor layers 38/39/40 are etched away from the uncovered portion until the p-type GaAs cap layer 35 is exposed. The depth is dependent of the etching time, and, accordingly, the wet etching is controlled with time. Then, the active region is planarized with the window region. Thus, the mesa structure is formed through the wet etching. This means that the location of the mesa structure is dependent on the controllability of the planarization. In detail, a dispersion of the relative relation between the mesa bottom in the window region and the mesa bottom in the active region is dependent on the dispersion of step between the window region and the active region upon completion of the planarization. The dispersion of the relative relation is causative of a dispersion of the coupling efficiency between the window region and the active region for the oscillation light in the horizontal transverse mode.
An actual dispersion is hereinbelow discussed. When the semiconductor laminated structure 38/39/40 is completed, the step takes between the window region and the active region, and the dispersion of the step over the semiconductor wafer is equivalent to the dispersion in the thickness of the compound semiconductor layers 38/39/40 or more serious than it. Upon completion of the wet etching, the dispersion of the step between the window region and the active region is further dependent on the dispersion of the depth over the semiconductor wafer, and is determined from both of the dispersion in the step and the dispersion of the depth.
Even though the etching stopper enhances the controllability of the wet etching and makes the dispersion of the depth insignificant, the dispersion of the thickness is still left on the semiconductor wafer, and the dispersion of the coupling efficiency takes places at the boundary between the window region and the active region in the horizontal transverse mode due to the dispersion of the thickness. This is the second problem.
Even if the window region is equalized in height with the active region, roughness is still left around the boundary between the window region and the active region. The rough surface results in roughness of the mesa bottom around the boundary upon completion of the mesa stripe through the etching. Moreover, the width of the SiO2 stripe 51 is not constant due to the irregularity of the thickness of the photo-resist mask formed over the rough surface. This results in that the mesa bottom is not constant in width. The roughness and the non-uniform width makes large scattering loss against the oscillation light in the horizontal transverse mode. This is the third problem.
It is therefore an important object of the present invention to provide a window type semiconductor laser light emitting device, which has a low coupling loss at the boundary between a window region and an active region in the transverse mode and a low scattering loss at the boundary.
It is also an important object of the present invention to provide a process for fabricating the window type semiconductor laser light emitting device.
In accordance with one aspect of the present invention, there is provided a window type semiconductor laser light emitting device comprising a substrate having a major surface, a laminated structure allowing electric current to flow therethrough and including a first clad layer formed over the major surface and having a first area and second areas on both sides of the first area and closer to the major surface than the first area, an active layer formed on the first area of the first clad layer and generating laser light from the electric current, a second clad layer formed on the active layer and forming a resonator together with the active layer and a part of the first clad layer having the first area and a third clad layer formed over the second clad layer and having side portions laterally projecting from both sides of the second clad layer and buried window layers formed of a first compound semiconductor material having large resistivity against the electric current on the second areas and under the side portions of the third clad layer and transparent to the laser light for providing window regions contiguous to the resonator, the buried window layers have upper surfaces substantially coplanar with an upper surface of the second clad layer, and a difference between the upper surfaces of the buried window layers and the upper surface of the second clad layer is equal to or less than 0.1 micron.
In accordance with another aspect of the present invention to provide a process for fabricating a window type semiconductor laser emitting device comprising the steps of preparing an epitaxial growing system having sources of elements, sources of dopant impurities and a source of inhibitor for restricting an epitaxial growth in a predetermined direction, epitaxially growing a first clad layer of a first conductivity type, an active layer and a second clad layer of a second conductivity type opposite to the first conductivity type on a major surface of a substrate of a first compound semiconductor material having a crystal plane close to a predetermined crystal plane, covering a first area of the second clad layer with an etching mask, selectively etching the second clad layer and the active layer until second areas of the first clad layer is exposed so as to form a step configuration having upper surfaces, first lower surfaces, second lower surfaces on both sides of the first lower surfaces and side surfaces formed between the upper surfaces and the first and second lower surfaces, epitaxially growing a second semiconductor material on the side surfaces so as to fill grooves defined by the upper surfaces, the first and second lower surfaces and the side surfaces by regulating the inhibitor, and completing a window type semiconductor laser light emitting device.